The invention relates to a method for producing a heat exchanger.
For many years, air-cooled heat exchangers have been used particularly in power plants for re-cooling of steam. These heat exchangers are configured as rows of heat exchanger bundles arranged in an A-shape, where the steam is condensed inside pipes. Heat transfer to the ambient air is improved by fins connected with the pipes. To guarantee a service life of at least several decades for these air-cooled heat exchangers, corrosion resistance is important. Several attempts have been made to construct the heat exchanger pipes of heat exchangers to resist corrosion. For example, U.S. Pat. No. 5,042,574 discloses to connect flat pipes plated with aluminum with corrugated folded aluminum fin webs in an annealing furnace by using an aluminum-silicon solder. Disadvantageously, this type of soldered connection can only be attained by using aluminum-plated flat pipes or by using plated aluminum fins. In addition to the comparatively complex process based on diverse materials, the flat pipes which are circumferentially closed by at least one longitudinal weld seam must not be plated with aluminum in the weld zone, because otherwise a trouble-free weld cannot be guaranteed. Brazing flat pipes made of steel to folded aluminum fin strips is problematic because soldering must be performed at relatively high temperatures of the order of around 600° C., i.e., close to the softening temperature of aluminum. The required solder typically consists of an aluminum-silicon eutectic having a melting point slightly below the softening point of aluminum. Selecting the flux which has to remove the oxide layers of the connection areas before the solder melts, but also becomes a liquid close to the softening temperature, also poses problems. The correct temperature profile for soldering can therefore be often only determined empirically.
Because aluminum and steel have different thermal expansion coefficients, the high soldering temperatures and subsequent cooling to ambient temperature can result in large material stresses, causing distortion of the connected parts and possible breakage at the solder joint, because the aluminum plating may not have been applied flawlessly or because a steel-aluminum intermediate layer may have formed between the steel pipe and the aluminum plating, as a result of melting of the aluminum layer during the brazing operation.
EP 1 250 208 B1 proposes to reduce the soldering temperature from conventionally about 600° C. to a range between 370° C. and 470° C. by using zinc-aluminum-alloys and special fluxes based on cesium-aluminum-tetrafluoride. The lower temperatures also cause less material stress; however, because of the heavy metal fraction special protective measures must be taken to handle the flux in order to prevent contamination of the environment.